Method and arrangement for symbol mapping in communication systems utilizing ofdm-techniques

ABSTRACT

The present invention relates to a method and arrangement for symbol mapping in wireless communication systems utilizing OFDM transmission technology in combination with advanced coding schemes. In the method according to the present invention, adapted for use in a wireless communication system utilising OFDM transmission technology, an OFDM grid is defined by at least a first dimension and a second dimensions from the dimensions time, frequency or space. The advanced coding scheme, for example Alamouti coding, outputs symbols that are related via the coding. At least some of the symbols, defining a code related symbol group should experience identical, or at least very similar, radio channels. According to one embodiment of the present invention, symbols from the same code related symbol group is placed as close together in the OFDM grid as possible.

This application is a continuation of U.S. patent application Ser. No.12/520,220 filed Jun. 19, 2009, which is a National Phase application ofPCT application PCT/SE06/050612 filed Dec. 21, 2006, the entire contentsof each of which are incorporated by reference herein.

BACKGROUND

1. Field of Invention

The present invention relates to symbol mapping in wirelesscommunication systems. In particular, the present invention relates tosymbol mapping in systems utilizing OFDM and advanced coding schemes.

2. Background of the Invention

Orthogonal Frequency Division Multiplexing (OFDM) has become awidespread technique in several wide-band digital communication systems.OFDM-techniques are used both in wire-bound systems such as ADSL andwireless systems such as WLAN. Recently OFDM has been selected as one ofthe transmission techniques in the next generation of mobilecommunication, 4G. OFDM offers high spectral efficiency and is fairlyrobust to interference and multipath propagation. Additionally OFDM canadapt to varying channel conditions without complex equalization.

In systems using OFDM, the symbols are. after channel coding andmodulation mapped onto a two-dimensional time-frequency grid. This gridmight also have a third dimension, the space dimension, if a wirelesssystem utilizing multiple transmit antennas. The grid or matrix, whichis a characteristic of the OFDM-techniques, defines resource-units whichare the smallest addressable entities in the system. Illustrated in FIG.1 a is a two-dimensional OFDM grid with a vertical frequency axis, and ahorizontal time axis. Four different frequencies, f₁-f₄, and seventime-slots. t₁-t₇. are utilized, each frequency/time-slot pairrepresenting one resource-unit, giving 28 individually addressableresource-units. In FIG. 1 b a space dimension is added, in the form of a4 antenna scenario, illustrating the spreading of signals on the 4antennas. In the following, for the reason of clarity, only thefrequency-time grid is illustrated, although the space dimension ispresent in implementations using multiple antennas.

The coded and modulated symbols are conventionally mapped on the OFDMresource-units in an ordered fashion. The mapping can be done in severaldifferent ways. Firstly, the mapping can be done row-wise orcolumn-wise. Secondly, the rows can be filled either from left-to-rightor from right-to-left, and thirdly, the columns can either be filledtop-to-bottom or bottom-to-top, totally giving eight different ways ofmapping. One example is given in FIG. 1 a wherein the mapping has beenperformed row-wise (frequency), left to right and top to bottom.

In combination with spectrum effective transmission technologies such asOFDM. the high speed communication envisaged with present and futurecommunication systems relies on advanced coding schemes. The purpose ofthe coding schemes is to improve the reliability of the data transfer,and hence reduce the number of retransmissions (lower Bit Error Rate(BER)). The redundancy introduced with the coding generally reduces thetransmission rate compared to an ideal loss-less transmission. However,the effective rate is, if an appropriate coding is utilized, improved.The novel coding schemes often referred to as space-time block codes(STBCs). offer redundancy with little effect on the ideal rate. The STBCcoding scheme that has received most attention, the Alamouti code, “ASimple Transmit Diversity Technique for Wireless Communications” IEEEJournal on Selected Areas in Communication, vol. 16, no. 8, October1998, pp. 1451-1458, offers full-rate, i.e. coding redundancy withoutreducing the rate. According to the coding scheme two symbols arejointly coded and transmitted over two resource-units. i.e. over twosub-carriers or two time slots. The performance depends on thesimilarity, as regards to channel conditions, between the tworesource-units. Other advanced coding schemes have similar requirements.

As certain relations between resource-units, for example as identicalradio channel conditions as possible, can be a prerequisite for a goodperformance of the coding, advanced coding schemes such as the Alamouti,inflicts requirements on the mapping of symbols on the OFDM grid.

SUMMARY OF THE INVENTION

OFDM transmission technology and advanced coding schemes such asspace-time block codes is capable of providing very high data rates inwireless systems. However, the technologies have to interact in aconstructive way, which has not always been the case in prior arttechniques.

The object of the present invention is to provide a method andarrangement that overcome the drawbacks of the prior art techniques.This is achieved by the method and the transmitting node, as defined inthe claims.

In the method according to the present invention, adapted for use in awireless communication system utilising OFDM transmission technology, anOFDM grid is defined by at least a first dimension and a seconddimensions from the dimensions time, frequency or space. Advanced codingschemes produce symbols that are related through the coding. Accordingto the invention the mapping of symbols related through the coding isdependent on the preferences set forth by the coding scheme. Some of thesymbols related through the coding should experience identical, or atleast similar, radio channels in order for the coding to be optimized.These symbols, defining a code related symbol group, is according to anembodiment of the invention, placed as close together in the OFDM gridas possible. Examples of coding schemes include, but is not limited toSTBC coding schemes, such as the Alamouti coding scheme.

The method according to the invention, comprises the main steps of:

-   -   placing a first symbol from a first code related symbol group at        a first position in the grid;    -   placing, if a second symbol belongs to the same code related        symbol group as the first symbol, the second symbol at a second        position as close to the position of the first symbol as        possible.

If the second symbols does not belong to the same code related symbolgroup as the first symbol, the second symbol can be placed freely in theOFDM grid. However, care should be taken to achieve an effective fillingof the grid. The second position may represent no change in onedimension and a change to an adjacent position in the other dimension.For example, if frequency is the first dimension, the second symbol isplaced at a position with an equal value in frequency as the firstsymbol, but with a different value in the other dimension, i.e. in adifferent time-slot.

Various versions, representing different embodiments of the invention,wherein apart from fulfilling the requirements set forth by the advancedcoding scheme, also care is taken to fill the OFDM grid in an effectiveway. According to one embodiment a row-wise mapping is utilized, a rowcorresponding to one dimension and a column to another dimension. Everyother row will be filled left-to-right and the other rows right-to-left.Alternatively a column-wise mapping is utilized, a row corresponding toone dimension and a column to another dimension, and every other columnshould be filled top-to-bottom and the other columns bottom-to-top.

The external radio environment could be such that a change of positionin one dimension is more critical than a change in another dimension. Ina multipath fading scenario, for example, a shift in frequency can givea dramatically different radio channel. In such case it is favourable totry to map symbols from one code related symbol group at positions inthe OFDM grid having the same frequency. For a mobile station, or userequipment, travelling at high speed it is favourable to transmit at thesame timeslot in order to keep the radio channel similar. According toone embodiment of the invention a preferred dimension is determinedbased at least partly on external radio environment factors, thepreferred dimension indicating that a change in that dimension wouldhave lower impact on the radio channel characteristics than in the otherdimension. In the mapping process it is prioritised to change value onlyin the preferred dimension for symbols of the same code related symbolgroup.

Thanks to the invention it is possible to optimize the combined usage ofadvanced coding schemes and OFDM transmission technology.

One advantage is that the method according to the invention is simple toimplement and does not require extensive processing resources.

A further advantage afforded by the inventive method is that externalradio environment factors can be taken into account in the mappingprocess.

Embodiments of the invention are defined in the dependent claims. Otherobjects, advantages and novel features of the invention will becomeapparent from the following detailed description of the invention whenconsidered in conjunction with the accompanying drawings and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in detail with reference to thedrawing figures, wherein

FIG. 1 a-b schematically illustrates the mapping of symbols on a OFDMgrid according to prior art mapping schemes in (a) two dimensions,frequency and time, and (b) three dimensions, frequency, time and space;

FIG. 2 a-b schematically illustrates the problems arising under certainconditions using prior art mapping schemes;

FIG. 3 is a flowchart over the method according to the presentinvention;

FIG. 4 is a flowchart over one embodiment of the method according to thepresent invention;

FIG. 5 a-c schematically illustrates the mapping produced by the methodaccording to the present invention;

FIG. 6 is a flowchart over one embodiment of the method according to thepresent invention;

FIG. 7 schematically illustrates a transmitting node according to thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

In a communication system wherein the present invention may be utilized,a plurality of entities share communication resources. The communicationsystem may be of the cellular type and one radio node being a basestation (BS) in communication with a plurality of mobile stations (MS),or user equipment (UE), as current and future versions of UMTS. Suchsystem may also comprise mobile and fixed relay stations. Other systemsalso sharing radio resources include different kinds of ad hoc systemsand multihop systems. wherein user equipment typically also acts asrelay stations forwarding a message towards an end node. Such systemsnormally have much less order structure than the cellular system. In thefollowing the method and arrangement according to the present inventionwill be exemplified in a cellular (wireless) scenario. However, asappreciated by the skilled in the art the invention may, with onlyslight modifications. be applicable also other type of communicationsystems wherein the communication resources are shared between aplurality of users, for example ADSL/VDSL-based systems and WLANsystems.

In the wireless system using OFDM, the symbols are mapped onto a two- orthree-dimensional grid as described with reference to FIG. 1 a-b forexample row-wise left-to-right and top to bottom.

Some advanced coding schemes, e.g. the Alamouti code, which will be usedas a non-limiting example, puts restrain on how the mapping should beperformed. By the Alamouti code two incoming symbols are coded into fouroutgoing sub-symbols and that the four sub-symbols are mapped onto two,preferable uncorrelated, antennas and two resource-units. Theresource-units should experience the same radio channel, or at least assimilar radio channel as possible. The two sub-symbols that are mappedon the same resource-unit are hereafter referred to as a symbol. Theradio channel will be as similar as possible if these symbols are placedas close as possible to each other in the time-frequency OFDM grid. Ifthe symbols are placed with a too large distance in the grid, theperformance in terms of bit error rate, block error rate, throughput anddelay will be reduced.

The prior art mappings described above are not suitable if the number ofsymbols that are coded together is not a divisor of the number ofsymbols that will fit into one row, in the case of row-wise mapping, orin one column, in the case of column-wise mapping, as illustrated inFIG. 2 a, wherein the “pairs” resulting from the Alamouti coding isdenoted 1 a-1 b, 2 a-2 b, etc. In the exemplary figure some symbol pairsare mapped closely together, for example symbols 1 a and 1 b and symbols2 a and 2 b. Due to the mismatch between the available positions(resource-units) in a row and the paired symbols, some of the symbolpairs are not mapped close to each other, symbols 4 a and 4 b, andsymbols 11 a and 11 b.

Another problem occurs when e.g. reference symbols (or pilot symbols) orcontrol symbols, are punctured into the OFDM grid. In the example shownin FIG. 2 b, the number of symbols coded together is a divisor of thenumber of symbols per row. Due to the positions used by other type ofsymbols, not all jointly coded symbols can be placed in positions closeto each other.

According to the method of the invention symbols are mapped on the OFDMgrid in a manner that optimizes the performance of the advanced codingscheme as much as possible. In the method at least a first and a seconddimensions are defined, for example frequency and time, forming an atleast two-dimensional grid. The symbols to be mapped on the grid, aregrouped according to the output from the coding scheme used. Symbolsthat have a coding relation to each other and requiring similar radiochannels, are referred to as a code related symbol group. If Alamouticode is used the code related symbol group takes up two positions in thegrid. It should be noted that, as described above, the coding scheme mayproduce output sub-symbols that should be placed in a third, preferablyuncorrelated, dimension, typically the spatial dimension. The mapping ofsymbols is illustrated in the flowchart of FIG. 3. The Alamouti codingroutine, preceding the steps of the method outputs a sequence ofsubsequent symbols to be placed in the OFDM grid. The method comprisesthe steps of:

-   305: Placing a first symbol at a first position in the grid.-   310: Selecting a subsequent second symbol.    -   315/325: If the second symbol belongs to the same code related        symbol group as the previous symbol, placing, the second symbol        at a second position as close to the position of the previous        symbol in the same code related symbol group as possible.    -   315/320: If the second symbol does not belong to the same code        related symbol group as the previous symbol, placing, the second        symbol “freely” in the OFDM grid. However, care can be taken to        get an ordered filling of the grid.

Steps 310-325 are repeated with the next subsequent symbol until thegrid is filled.

The placing of the second symbol at a second position in step 325 ispreferably performed so that the second position represents no change inone dimension and a change to an adjacent position in the otherdimension. If that is not possible, a next nearest position is chosen,and so on. For example, if frequency is the first dimension, the secondsymbol is placed at a position with an equal value in frequency as thefirst symbol, but with a different value in the other dimension, i.e. ina different time-slot.

The above will result in a pseudo random-walk, that might be unfavorablein terms of effectively filling the grid. The method may therefore bemodified according to, illustrated in the flowchart of FIG. 4:

-   402: Selecting/determining a first preferred dimension, e.g. time,    and a first preferred direction associated to the first dimension,    e.g. increasing, and a second preferred direction, e.g. decreasing    associated with the second dimension, e.g. frequency.-   405: Placing a first symbol at a first selected position in the    grid, the position for example representing the lowest value in the    first dimension and the highest value in the second dimension.-   410: Selecting a subsequent second symbol.-   415/420: Placing, if possible, the next symbol in the next position    in the preferred direction in the first preferred dimension (the    next time-slot).-   425/430: If the next symbol was not possible to position adjacent to    the first symbol in the first preferred dimension and in the    preferred first direction (time), placing the second symbol in the    next position in the second preferred direction (decrease), i.e. a    move in the second dimension (frequency).-   435: Placing the second symbol at a position as close to the first    position as possible.-   440/445: If not possible to move in the first preferred direction    reverse the first preferred direction.

Steps 410-445 are repeated with the next subsequent symbol until theOFDM grid is filled.

The above described method of mapping can be envisaged as changing thedirection of filling the symbols into the two-dimensional grid ifneeded. If row-wise mapping is used, every other row will be filledleft-to-right and the other rows right-to-left. If column-wise mappingis used, every other column should be filled top-to-bottom and the othercolumns bottom-to-top. This will also give eight different ways of doingthe mapping, in the same way as described for the prior art mapping. Theresult of such mapping is illustrated in FIG. 5 a, wherein the symbolsare coded together two and two, and were all the symbol pairs are mappedclose to each other using the invented mapping. The mapping is donerow-wise, starting left-to-right, alternating with right-to-left, andtop-to-bottom.

In a situation as described with reference to FIG. 2 b, whereinpositions are reserved for certain type of signaling the embodimentdescribed with reference to the flowchart of FIG. 4 would give the gridillustrated in FIG. 5 b. Other constrains could be introduced in themethod to prefer a next nearest position over a nearest, if such achoice gives a more effective filling of the grid. The result of such amodification, representing an embodiment of the method, is illustratedin FIG. 5 c. A similar approach is to introduce a constrain that therow-wise (or column-wise) back and forth filling should be keptregardless of the reserved positions.

Under certain circumstances it may be preferred that the placing ofsymbols in one code related symbol group is constant with respect to onedimension in the grid. For example, that all symbols in a code relatedsymbol group is given the same frequency or timeslot. This could be thecase if the UE is traveling at relatively high speed. In order tosatisfy the requirement of similar radio channels for all symbols,according to the Alamouti code, the placing of symbols in a code relatedsymbol group should preferably be within the same timeslot, i.e. acolumn-wise mapping if referring to the exemplary illustrations of FIG.5 a-c. Another scenario is in a situation wherein the UE is in ageographical position characterized by complex multiple path fading. Insuch a scenario a constant frequency for all symbols in the code relatedsymbol group is preferred, corresponding to a row-wise mapping in FIG. 5a-c. The preferences due to the radio conditions exemplified above, willbe referred to as “external radio environment factors”.

A modification of the method, representing an embodiment of theinvention, taking the external radio environment factors into account,is illustrated in the flowchart of FIG. 6, and comprises of the steps:

-   601: Determining the external radio environment factors, i.e. if the    radio conditions between the BS and the UE is such that a change in    one of the dimension would cause lower impact on the characteristics    of the radio channel, than in another.-   602: Selecting/determining a first preferred dimension. e.g. time,    based on the external radio environment factors and a first    preferred direction associated to the first dimension, e.g.    increasing, and a second preferred direction, e.g. decreasing    associated with the second dimension, e.g. frequency. The first    preferred dimension should correspond to the dimension wherein the    change of position causes the lowest impact on the characteristics    of the radio channel.-   610: Select group of symbols, i.e. all symbols of a code related    symbol group.-   615: Search the grid, for example starting from the position    representing the lowest value in the first dimension and the highest    value in the second dimension, after a number of positions changing    only in the preferred dimension, the number of positions    corresponding to the number of symbols in the code related symbol    group. Preferably the positions are consecutive, to minimize the    change also in the preferred dimension.-   620/625: Placing, if required number of positions in the preferred    dimension, were identified, the symbol of the code related symbol    group in the identified positions.-   620/630: If not possible to find the required number of positions    changing only in the preferred dimension, place the symbols of the    code related symbol group as close in the second dimension as    possible.

The steps 610-630 are repeated with the next code related symbol groupuntil the grid is filled.

The external radio environment factors can be determined by knownmethods. In fact, the external radio environment factors are in manycases already present in the BS, for example, as a result ofcharacterizing the radio channels and/or estimating the speed of an UEfor other purposes.

If, for example a UE is traveling at high speed while maintaining acommunication, the radio channel changes rapidly in time. Hence, inorder to maintain a similar radio channel over the symbols in a coderelated symbol group, the symbols should preferably be transmitted onpositions in the OFDM grid sharing the same timeslot, but with differentfrequency. In this case the dimension corresponding to frequency is thepreferred dimension.

In a multipath fading scenario, the radio channel may vary considerablyeven for minor changes in frequency. In this case it is preferably totransmit all symbols of a code related symbol group on the samefrequency, hence time is determined to be the preferred dimension.

Arrangements according to the present invention in a transmitting nodesuitable for effectuating the above described embodiments areschematically illustrated in FIG. 7. The modules and blocks according tothe present invention are to be regarded as functional parts of a basestation and/or an user equipment in a communication system, and notnecessarily as physical objects by themselves. The modules and blocksare preferably at least partly implemented as software code means, to beadapted to effectuate the method according to the invention. The term“comprising” does primarily refer to a logical structure and the term“connected” should here be interpreted as links between functional partsand not necessarily physical connections. However, depending on thechosen implementation, certain modules may be realized as physicallydistinctive objects in a receiving or sending node. The term“transmitting node” should be given a broad interpretation, meaning anydevice, stand alone, or part of a greater entity, capable of wirelesscommunication utilizing OFDM-technology. Examples of transmitting nodesinclude, but is not limited to radio base stations, user equipments suchas mobile phones, PDAs and laptop computers.

The transmitting node 705 comprises radio communication means 710, whichprovides the necessary functionalities for performing the actualreception and transmission of radio signals and is well known by theskilled person. The radio communications means 710 are adapted tocommunicate utilizing OFDM technology and preferably utilizing multipleantennas 715. According to the invention the transmitting node 705 isprovided with a coding module 720 adapted to code an incoming stream ofsymbols, for example with Alamouti coding, producing an output stream ofsymbols, comprising groups of symbols that have a code relationrequiring the symbols to be transmitted under very similar conditions,code related symbol groups. A mapping module 725 is in connection withthe radio communication means 710 and the coding module 720. The mappingmodule 725 is adapted to receive the outputted stream of symbols and tomap symbols on the OFDM grid. According to the invention the mappingmodule 725 is adapted to map symbols from code related symbol groups ina manner that is dependent on preferences set forth by the codingscheme. According to one embodiment of the invention the coding module720 performs a STBC, e.g. Alamouti coding, and the mapping module 725 isadapted to position symbols of the same code related symbol group asclose to each other as possible in the OFDM grid. Further adaptations ofthe mapping module 725 corresponds to the different embodiments of themethod described above.

According to a further embodiment the transmitting node 705 is providedwith an external radio environment determination module 730 adapted tocharacterize the radio environment and other radio nodes, referred to asreceiving nodes, that the transmitting node 705 is in communicationwith. The external radio environment determination module 730 is inconnection with the radio communication means for receiving datarelating to the radio environment. Based on the radio environment andthe characteristics of a receiving node, the external radio environmentdetermination module 730 is adapted to provide the mapping module 725with a preferred first dimension, and possibly a preferred direction inthat dimension, the preferred dimension representing the dimension forwhich a change has the lowest impact on the radio channel. The externalradio environment determination module 730, may include, or be inconnection to, means for determining the speed and/or direction of areceiving node.

While the invention has been described in connection with what ispresently considered to be the most practical and preferred embodiments,it is to be understood that the invention is not to be limited to thedisclosed embodiments, on the contrary, is intended to cover variousmodifications and equivalent arrangements within the appended claims.

What is claimed is:
 1. A mapping method, in a transmitting node for usein a wireless communication system, for mapping symbols on an OFDM grid,the grid defined by at least a first dimension and a second dimensionfrom the dimensions time, frequency and space, and wherein a codingscheme is used giving a plurality of code related symbol groups, eachcomprising a plurality of symbols that are related through the codingscheme, the method comprising using an alternating mapping direction forsymbol groups, such that consecutive symbols of each said code relatedsymbol group are mapped as close to each other as possible in the OFDMgrid.
 2. The mapping method according to claim 1, wherein using analternating mapping direction for symbol groups comprises: placing afirst symbol from a first code related symbol group at a first positionin the grid; and placing, if a second symbol belongs to the same coderelated symbol group as the first symbol, the second symbol at a secondposition as close to the position of the first symbol as possible. 3.The mapping method according to claim 2, wherein the second positionrepresents no change in one dimension and a change to an adjacentposition in the other dimension as regards to the first position.
 4. Themapping method according to claim 2, wherein the coding scheme is anAlamouti coding scheme and each code related symbol group comprises twojointly coded symbols, the two symbols from the code related symbolgroup placed as close to each other as possible in the OFDM grid.
 5. Themapping method according to claim 1, wherein using an alternatingmapping direction for symbol groups comprises: selecting a firstpreferred dimension as the first dimension and a first preferreddirection associated to the first dimension, and a second preferreddirection associated with the second dimension; placing a first symbolat a first selected position in the grid; placing, if possible, a secondsymbol in the next position in the preferred direction in the firstpreferred dimension; if the second symbol was not possible to placeadjacent to the first symbol in the first preferred dimension and in thepreferred first direction, placing the second symbol in the nextposition in the second preferred direction; and if not possible to movein the grid in the first preferred direction, reversing the firstpreferred direction.
 6. The mapping method according to claim 1, whereina row-wise mapping is utilized, a row corresponding to one dimension anda column to another dimension, and wherein every other row is filledleft-to-right and the other rows right-to-left.
 7. The mapping methodaccording to claim 1, wherein a column-wise mapping is utilized, a rowcorresponding to one dimension and a column to another dimension, andwherein every other column is filled top-to-bottom and the other columnsbottom-to-top.
 8. The mapping method according to claim 1, furthercomprising determining an external radio environment factor, andcarrying out said alternate mapping in dependence on the external radiofactor by selecting at least one of a preferred dimension and anassociated preferred direction, in dependence on the external radioenvironment factor.
 9. The mapping method according to claim 8, whereinthe selection of a preferred dimension is based on the external radioenvironment factor, and the selection is such that a change in thepreferred dimension would have the lower impact on radio channelcharacteristics as compared to a change in another dimension.
 10. Themapping method according to claim 9, wherein the transmitting node, anassociated receiving node, or both, are experiencing time-varying radiochannels, and further comprising selecting frequency as the preferreddimension.
 11. The mapping method according to claim 9, wherein thetransmitting node, an associated receiving node, or both, areexperiencing multipath fading, and further comprising selecting time asthe preferred dimension.
 12. The mapping method according to claim 9,further comprising: determining external radio environment factors;selecting a first preferred dimension, based on the external radioenvironment factors and a first preferred direction associated with thefirst dimension, the first preferred dimension selected as the dimensionwherein the change of position causes the lowest impact on thecharacteristics of the radio channel; selecting a group of symbolscomprising all symbols of a code related symbol group; searching thegrid for a number of positions changing only in the preferred dimension,the number of positions corresponding to the number of symbols in thecode related symbol group; and placing, if the required number ofpositions in the preferred dimension were identified, the symbols of thecode related symbol group in the identified positions.
 13. The mappingmethod according to claim 12, further comprising: placing, if notpossible to find the required number of positions, the symbols of thecode related symbol group as close together in the second dimension aspossible.
 14. The mapping method according to claim 12, whereinsearching the grid for the number of positions changing only in thepreferred dimension comprises searching for consecutive positions. 15.The method according to claim 1, wherein one of the first and seconddimensions is frequency and the other dimension is time.
 16. Atransmitting node configured for use in a wireless communication systemutilizing OFDM transmission technology, the transmitting nodecomprising: a radio transmitter adapted to transmit radio signals usingmultiple antennas; a coding module adapted to code an incoming stream ofsymbols thereby producing an output stream of symbols comprising coderelated symbol groups, each code related symbol group having a coderelation; a mapping module adapted to control mapping of the outputtedstream of symbols on an OFDM grid, for transmission by the radiotransmitter, wherein the mapping module is adapted to map the coderelated symbol groups in alternating mapping directions, such thatconsecutive symbols from each code related symbol group are mapped asclose to each other as possible in the OFDM grid.
 17. The transmittingnode according to claim 16, wherein the coding module is adapted toperform a STBC, and the mapping module is adapted to position symbols ofthe same code related symbol group as close to each other as possible inthe OFDM grid.
 18. The transmitting node according to claim 16, whereinthe mapping module is adapted to place a first symbol from a first coderelated symbol group at a first position in the OFDM grid; subsequentlyplace a second symbol, if it belongs to the same first code relatedsymbol group, and if possible, at a second position that represents anequal value as that of the first position in one dimension of the OFDMgrid and an adjacent value in the other dimension of the OFDM grid. 19.The transmitting node according to claim 16, wherein the mapping moduleis adapted to use a row-wise mapping, a row corresponding to onedimension of the OFDM grid and a column to another dimension of the OFDMgrid, and wherein every other row is filled left-to-right and the otherrows right-to-left.
 20. The transmitting node according to claim 16,wherein the mapping module is adapted to use a column-wise mapping, arow corresponding to one dimension of the OFDM grid and a column toanother dimension of the OFDM grid, and wherein every other column isfilled top-to-bottom and the other columns bottom-to-top.
 21. Thetransmitting node according to claim 16, further comprising an externalradio environment determination module in connection with the radiotransmitter and the mapping module, the external radio environmentdetermination module adapted to characterize the radio environmentbetween the transmitting node and a receiving node that the transmittingnode is in communication with, and adapted to, based on thecharacterization, provide the mapping module with a preferred firstdimension of the OFDM grid, the preferred first dimension representingthe dimension of the OFDM grid in which a change has the lowest impacton the radio channel characteristics.
 22. A method in a transmittingnode of mapping symbols on an OFDM grid for transmission, said OFDM gridhaving at least two dimensions from among a time dimension, a frequencydimension, and a spatial dimension, the method comprising: identifying apreferred dimension of the OFDM grid based at least in part on radiochannel characteristics between the transmitting node and an associatedreceiving node, said preferred dimension identified as the dimensionwherein the radio channel characteristics change less for changes ingrid position along the preferred dimension; generating symbols for OFDMtransmission via an encoding scheme that produces code related symbolgroups; and preferentially placing individual symbols on the OFDM gridaccording to their corresponding symbol related group by placing eachsymbol in a code related group on the OFDM grid such that grid positionsfor those symbols change only in the preferred dimension.